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Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) Contract | Sep. 30, 2019 USD ($) Contract | Sep. 30, 2020 USD ($) Contract | Sep. 30, 2019 USD ($) Contract | Dec. 31, 2019 USD ($)
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Number of Contracts | Contract | 17 | 6 | 33 | 26 |
Total Pre-Modification Outstanding Recorded Investment | $ 1,309 | $ 217 | $ 1,781 | $ 7,381 |
Post- Modification Outstanding Recorded Investment | 1,165 | 215 | 1,607 | 6,917 |
Troubled Debt Restructuring [Policy Text Block] | 489 | 0 | 489 | 398 |
Extended Maturity [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Total Pre-Modification Outstanding Recorded Investment | 0 | 0 | 0 | 17 |
Contractual Interest Rate Reduction [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Total Pre-Modification Outstanding Recorded Investment | 705 | 32 | 776 | 149 |
Contractual Payment Modification [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Total Pre-Modification Outstanding Recorded Investment | $ 604 | $ 185 | $ 1,005 | $ 7,215 |
Loans to individuals [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Number of Contracts | Contract | 4 | 2 | 14 | 7 |
Total Pre-Modification Outstanding Recorded Investment | $ 67 | $ 37 | $ 263 | $ 98 |
Post- Modification Outstanding Recorded Investment | 63 | 34 | 245 | 87 |
Troubled Debt Restructuring [Policy Text Block] | 0 | 0 | 0 | 0 |
Loans to individuals [Member] | Extended Maturity [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Total Pre-Modification Outstanding Recorded Investment | 0 | 0 | 0 | 0 |
Loans to individuals [Member] | Contractual Interest Rate Reduction [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Total Pre-Modification Outstanding Recorded Investment | 43 | 0 | 114 | 0 |
Loans to individuals [Member] | Contractual Payment Modification [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Total Pre-Modification Outstanding Recorded Investment | 24 | 37 | 149 | 98 |
Acquired Loans [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 3,306 | 7,481 | 3,447 | 5,031 |
Impaired Financing Receivable, Interest Income, Accrual Method | 77 | 1 | 94 | 24 |
Related allowance | 0 |  | 0 |  | $ 0
Recorded investment | 2,055 |  | 2,055 |  | 2,519
Unpaid principal balance | 2,447 |  | 2,447 |  | 2,993
Acquired Loans [Member] | Commercial Loan [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 74 | 4,697 | 74 | 2,255 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Recorded investment | 74 |  | 74 |  | 73
Unpaid principal balance | 74 |  | 74 |  | 73
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 308 | 0 | 137 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Recorded investment | 308 |  | 308 |  | 0
Unpaid principal balance | 308 |  | 308 |  | 0
Acquired Loans [Member] | Residential Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 1,473 | 1,950 | 1,772 | 1,966 |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 | 1 | 18 | 6 |
Recorded investment | 1,429 |  | 1,429 |  | 2,136
Unpaid principal balance | 1,794 |  | 1,794 |  | 2,585
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 1,440 | 666 | 1,453 | 636 |
Impaired Financing Receivable, Interest Income, Accrual Method | 76 | 0 | 76 | 18 |
Recorded investment | 233 |  | 233 |  | 298
Unpaid principal balance | 257 |  | 257 |  | 320
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 11 | 13 | 11 | 14 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Recorded investment | 11 |  | 11 |  | 12
Unpaid principal balance | 14 |  | 14 |  | 15
Acquired Loans [Member] | With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 3,306 | 7,326 | 3,447 | 4,871 |
Impaired Financing Receivable, Interest Income, Accrual Method | 77 | 1 | 94 | 24 |
Recorded investment | 2,055 |  | 2,055 |  | 2,519
Unpaid principal balance | 2,447 |  | 2,447 |  | 2,993
Acquired Loans [Member] | Real Estate Construction With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Acquired Loans [Member] | Residential Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 155 | 0 | 160 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Acquired Loans [Member] | Loans To Individuals With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Acquired Loans [Member] | With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 155 | 0 | 160 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Originated Loans [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 50,770 | 27,987 | 47,310 | 26,858 |
Impaired Financing Receivable, Interest Income, Accrual Method | 127 | 131 | 403 | 474 |
Related allowance | 7,418 |  | 7,418 |  | 2,432
Recorded investment | 47,673 |  | 47,673 |  | 29,698
Unpaid principal balance | 58,556 |  | 58,556 |  | 39,122
Originated Loans [Member] | Commercial Loan [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 6,423 | 4,677 | 4,569 | 4,064 |
Impaired Financing Receivable, Interest Income, Accrual Method | 16 | 8 | 45 | 36 |
Related allowance | 1,497 |  | 1,497 |  | 1,580
Recorded investment | 4,750 |  | 4,750 |  | 8,290
Unpaid principal balance | 12,778 |  | 12,778 |  | 10,032
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 1,918 | 1,902 | 4,062 | 2,129 |
Impaired Financing Receivable, Interest Income, Accrual Method | 10 | 3 | 15 | 10 |
Recorded investment | 795 |  | 795 |  | 1,848
Unpaid principal balance | 987 |  | 987 |  | 6,997
Originated Loans [Member] | Real Estate Construction With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 18 | 0 | 6 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Recorded investment | 54 |  | 54 |  | 0
Unpaid principal balance | 53 |  | 53 |  | 0
Originated Loans [Member] | Residential Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 10,198 | 10,254 | 10,380 | 10,751 |
Impaired Financing Receivable, Interest Income, Accrual Method | 72 | 86 | 235 | 280 |
Recorded investment | 9,962 |  | 9,962 |  | 10,372
Unpaid principal balance | 11,965 |  | 11,965 |  | 12,437
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 14,312 | 3,582 | 13,994 | 3,854 |
Impaired Financing Receivable, Interest Income, Accrual Method | 23 | 28 | 90 | 129 |
Recorded investment | 14,228 |  | 14,228 |  | 3,015
Unpaid principal balance | 14,523 |  | 14,523 |  | 3,210
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 494 | 389 | 469 | 356 |
Impaired Financing Receivable, Interest Income, Accrual Method | 3 | 4 | 9 | 11 |
Recorded investment | 495 |  | 495 |  | 406
Unpaid principal balance | 808 |  | 808 |  | 640
Originated Loans [Member] | With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 26,940 | 16,127 | 28,911 | 17,090 |
Impaired Financing Receivable, Interest Income, Accrual Method | 108 | 121 | 349 | 430 |
Recorded investment | 25,534 |  | 25,534 |  | 15,641
Unpaid principal balance | 28,336 |  | 28,336 |  | 23,284
Originated Loans [Member] | Real Estate Construction With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Originated Loans [Member] | Residential Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 740 | 0 | 347 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 1 | 0 | 6 |
Related allowance | 0 |  | 0 |  | 1
Recorded investment | 0 |  | 0 |  | 474
Unpaid principal balance | 0 |  | 0 |  | 498
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 17,407 | 6,443 | 13,830 | 5,357 |
Impaired Financing Receivable, Interest Income, Accrual Method | 3 | 1 | 9 | 2 |
Related allowance | 5,921 |  | 5,921 |  | 851
Recorded investment | 17,389 |  | 17,389 |  | 5,293
Unpaid principal balance | 17,442 |  | 17,442 |  | 5,308
Originated Loans [Member] | Loans To Individuals With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Originated Loans [Member] | With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 23,830 | 11,860 | 18,399 | 9,768 |
Impaired Financing Receivable, Interest Income, Accrual Method | 19 | $ 10 | 54 | $ 44 |
Related allowance | 7,418 |  | 7,418 |  | 2,432
Recorded investment | 22,139 |  | 22,139 |  | 14,057
Unpaid principal balance | $ 30,220 |  | $ 30,220 |  | $ 15,838

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